As an object moves farther from a camera, an angle that the object subtends at a photosensor on which the camera images the object decreases, and a size of an image of the object that the camera projects onto the photosensor and a number of pixels in the photosensor covered by the image decreases. With the decrease in image size and number of pixels onto which the image is projected, resolution of the object's features decreases and details of the object may become indiscernible. An optical system of a camera that provides zoom functionality is adjustable so that as an object recedes from the camera it may be “zoomed in” to conserve or increase an angle that the object subtends at the camera photosensor. Zooming in on an object magnifies an image of the object that the camera focuses onto its photosensor and improves resolution of imaged features of the object.
Zoom adjustment for a camera is typically provided by a mechanical system that moves a lens or lenses in the camera's optical system to change the relative positions of the lenses and thereby a focal length of the optical system. The system moves the lenses to provide the camera with a relatively long focal length and a field of view (FOV) characterized by a relatively small view angle to zoom in on an object and magnify an image of the object that the camera acquires. The system moves the lenses to provide the camera with a relatively short focal length and relatively wide-angle FOV to “zoom out” the object, and demagnify the object's image that the camera acquires.
The FOV of a camera is a region of space defined by a solid angle that extends from an optical center of the camera and for which points therein are imaged by the camera's optical system on the camera photosensor. Size of a FOV for most imaging purposes is conveniently measured by horizontal and vertical view angles. The horizontal and vertical view angles are largest angles between two lines that extend from the optical center of the camera, are contained in the FOV, and are coplanar with the camera optical axis in a plane respectively parallel and perpendicular to the ground.
Whereas zooming in magnifies images of objects in a scene that the camera images, it also decreases the view angles of the camera's FOV and as a result decreases a size of the imaged scene, and a portion of an environment surrounding the camera that the camera is able to image. Whereas zooming out demagnifies images of objects in a scene that the camera images, it also increases the view angles of the camera's FOV and as a result increases a size of the imaged scene and a portion of an environment surrounding the camera that the camera is able to image.
For many applications, such as for tracking a person's gestures to interface the person with a computer, it is advantageous that a camera that images the person image the person with an acceptable degree of resolution over a relatively large volume of space, hereinafter referred to as an “active space”. For example, to interface a person with a computer game, hereinafter also a full-body, three-dimensional (3D) game, such as a boxing game or a game requiring exercise, that responds to full body motion of the person, it can be advantageous that the camera image the person with acceptable resolution substantially everywhere in the active space. The active space may for example have a “length” that extends from a distance near the camera equal to about 1 m (meters) to a far distance from the camera equal to about 3 m. To provide advantageous image resolution over the length of the active space, the camera optics may be configured for zoom adjustment.
Imaging a person to track the person's gestures or motion during playing a 3D game is advantageously performed using a 3D camera, such as a triangulation or a time of flight (TOF) 3D camera, which acquires distances to features of the person and optionally features of the person's environment. The distances acquired by the 3D camera for the person and optionally the person's environment at a substantially same given time provides a “range image” of a scene comprising the person. 3D cameras are generally active illumination cameras that image a scene with light they generate and configure to determine distances to features in the scene.
A triangulation type 3D camera acquires distances to features in a scene from angles at which the camera images the features from two, generally slightly, different perspectives. The triangulation camera may illuminate the scene with spatially modulated light referred to as “structured light”. A time of flight (TOF) 3D camera acquires distances to features in a scene that the camera images by timing how long it takes temporally modulated light that it transmits to travel to the features and back to the camera. The camera transmits the light generally in very short light pulses and images light from the pulses that is reflected by the features that it collects to determine round trip, that is “back and forth”, travel times for light.
Providing a 3D camera with zoom optics is generally both technically and cost-wise challenging.